In recent years there is arising a demand for Zn-electroplated steel sheets with higher corrosion resistance especially from automobile industry circles. In compliance with this demand, improved Zn-Ni type alloy-plating of steel sheets such as Zn-Ni-alloy plating, Zn-Ni-Co-alloy plating, etc. has been developed.
Although steel sheets with such Zn-Ni type alloy electroplating is excellent in corrosion resistance in the bare state, they are inferior in the susceptibility to the phosphating (phosphate salt treatment) as the pretreatment for electrodeposition coating (electrophoretic painting). That is, uniform dense phosphate salt crystals are not formed on the plated surface, and therefore adhesion of the electrodeposited coating is not satisfactory.
There was previously developed improved Zn-Ni-alloy electroplated steel sheets, said alloy containing a slight amount of Ti, Co, etc. in order to overcome the above-mentioned problem, which is described in the copending Ser. No. 555,711. Said process comprises electroplating steel sheets in an acidic plating bath containing 10-40 g/l (gram as atom per liter bath) zinc, 15-160 g/l nickel, 0.2-10 g/l titanium, 0.1-5 g/l cobalt and 0.1-5 g/l aluminum or 0.2-4 g/l magnesium, the pH of which is 1.5-2.5.
The plated steel sheets obtained by this process have remarkably improved adhesion of electrodeposited coatings (electrophoretically painted layer) and are excellent in corrosion resistance in the bare state, since the plated layer contains a slight amount of Ti which is uniformly distributed in the plated layer and works as nuclei for crystalization of phosphate salts in the phosphating.
However, the adhesion of the plated layer to the substrate steel sheet in the above-mentioned plated steel sheets is not quite satisfactory. The plated layer on which a thick electrodeposited coating is applied is liable to be peeled off from the steel substrate by impulsive deformation, since the electrodeposited coating is dense and hard per se and is firmly bonded to the plated layer so that the latter suffers strong compressive stress. For instance, the plated layer on which an electrodeposited coating of more than 15.mu. is applied is easily peeled off when tested by a Du Pont impulse tester. Thus, the above-mentioned improved Zn-Ni-alloy-electroplated steel sheet is not quite satisfactory as a substrate for electrodeposition coating.
We proceeded with research for improving adhesion of said plated Zn-Ni alloy layer to the steel substrate and have found that the adhesion can be improved by pre-electroplating substrate steel sheets with a Zn-Ni alloy using an electroplating bath of a specific composition under specific conditions before effecting said Zn-Ni electroplating.